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From APUE

When we execute a program file, the effective user ID of the process is usually the real user ID. However, we can also set the set-user-ID bit in the file’s mode word (st_mode) that says, ‘‘When this file is executed, set the effective user ID of the process to be the owner of the file (st_uid).’’

For example, if the owner of the file is the superuser and if the file’s set-user-ID bit is set, then while that program file is running as a process, it has superuser privileges. This happens regardless of the real user ID of the process that executes the file. As an example, the UNIX System program that allows anyone to change his or her password, passwd(1), is a set-user-ID program. This is required so that the program can write the new password to the password file, typically either /etc/passwd or /etc/shadow, files that should be writable only by the superuser. Because a process that is running set-user-ID to some other user usually assumes extra permissions, it must be written carefully.

Assume the set-user-ID bit in a program file is set.

When does changing the process' effective user ID to be the owner of the file happen, before or after the kernel decides that the process can execute the file via the file access test?

  • If "before", since the process' effective user ID is changed to be the owner of the file first, doesn't the kernel always decide that the process can execute the file via the file access test?

  • If "after", since the kernel decides that the process can execute the file via the file access test, the process must have the effective ID matching the file's owner and the file also has the permission bit set. Then why is there a need to still change the effective user ID of the process to be the owner of the file?

Thanks.

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  • If your "after" assumption would be correct, how would you run programs not owned by yourself? Or in other words: file permissions define who is allowed to run a program, setuid defines under which uid it runs under.
    – nohillside
    CommentedApr 25, 2018 at 20:27
  • If "after", then the kernel already decides that the process can execute the file via the file access test, so we can deduce that the process must have the effective ID matching the file's owner and the file also has the permission bit set.
    – Tim
    CommentedApr 25, 2018 at 20:28
  • No. A file can be owned by root but you still can execute it if the access rights are r-xr-xr-x for instance.
    – nohillside
    CommentedApr 25, 2018 at 20:34
  • Then why is there a need to change the effective id of the process to be the owner of the file? Assume that you believe "after" is correct.
    – Tim
    CommentedApr 25, 2018 at 20:50
  • Tim, are you possibly thinking that the kernel code that does the access check is running with whatever limited permissions the user-mode process that called execve has? That is not the case; a kernel-mode process has permissions equal to or exceeding that of user-mode root, no matter what uid the user process is.
    – Mark Plotnick
    CommentedApr 25, 2018 at 21:14

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